1. Field of the Invention
The present invention relates to a lithographic apparatus including a motor cooling device, and a cooling device for removing heat from a motor.
2. Description of the Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Conventional lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
Patterning devices as well as substrates may be moved during the transfer of the pattern onto the substrate. For this purpose, patterning devices and substrates are mounted to associated supports called patterning support and substrate support, respectively, which are driven by motors in several degrees of freedom. Movements of supports are to be made in the shortest times possible. Therefore, motors for a substrate support, in particular, but not limited thereto, tend to require an increasing amount of power to allow increasing accelerations and decelerations, where also the mass to be accelerated and to be decelerated has a tendency to increase. At the same time, there is a need to keep the physical dimensions of the motors limited.
In operation, a motor dissipates heat. In an electric motor, heat is generated by a current flowing in an electrical coil accommodated in a part of the motor. The heat dissipated in a motor is to be removed through at least one cooling element, such as a cooling plate, mounted in a thermal contact with the heat generating part of the motor. In a cooling element, heat generated in a motor is transferred to a cooling fluid (a gas or a liquid, e.g. water). The cooling fluid is fed to the cooling element through a supply duct, flows into and through the cooling element, e.g. in channels provided in the cooling element, and out of the cooling element into a discharge duct, whereby heat is removed from the heat generating part of the motor. A portion of this heat is transferred from the cooling element to an environment of the motor, or vice versa, by radiation and convection, where the amount of heat transferred is determined by an average temperature of a surface of the cooling element. It is desirable to limit the heat transferred between the cooling element surface and the environment to maintain stable operating conditions of the apparatus containing the motor, even if the heat dissipated in the motor varies when the load of the motor varies.
Conventionally, a motor has been cooled by feeding a cooling fluid having a fixed temperature to the motor's cooling element with a fixed flow rate. Since the dissipated heat in the motor varies in time while the amount of water per unit of time does not vary, the average temperature of the surface of the cooling element will vary with the load of the motor. Since the average temperature and variations in this average temperature are to be limited, also the load of the motor (the heat dissipated in the motor) is limited.
With the increased power of motors, and the size limitations of the motors at the same time, problems arise in the cooling of the motors.
Increasing a cooling rate of a cooling fluid by increasing a cross-section of a cooling channel in a cooling element is undesirable since this will increase a magnetic gap in the motor, thereby deteriorating the motor constant.
Increasing a flow rate of a cooling fluid by increasing a flow speed of the cooling fluid in a cooling element is undesirable in view of the high pressure needed, and in view of the risk of the cooling fluid flow becoming turbulent instead of laminar, thus creating unwanted vibrations and an excessive pressure drop. A high pressure would further necessitate a construction capable of withstanding such pressure. Also, a high pressure or flow speed could give rise to an undesired generation of vibrations.
Decreasing a cooling fluid temperature, and allowing a higher temperature rise of the cooling fluid is undesirable in view of the resulting relatively high temperature fluctuations when the motor load varies over a broad range.